I am an experienced cyclist but new to tandems this year, and I have to say I have really enjoyed sharing cycling with my family.

Here is my long question:

I am riding a KHS Alite (40 pounds?) that is pretty heavily loaded. Up front we have a child carrier on teh top tube + my son (37+ pounds together), me (175), my wife (125), and my 6 year old on an adams out the back (he weighs 47 pounds and the adams feels like it weighs at least the same. Maybe we are not that heavy in the realm of tandems.

The KHS comes with 1.75 tires that we have been riding. Today I tried some 2.0 slicks (moby bites) that I had in teh garage. I am thinking of throwing some higher pressure skinny slicks (IRC smoothies at 95 PSI) on the bike, but because I am carrying my children I am nervous about the load on the tires.

Does anyone have experience with running thinner tires with heavy loads? I am also considering picking up some 1.4 Ritchey tom slicks or IRC metro duros instead of the smoothies, but I figured If I have the smoothies and they would work, why not use them.

Your rim is the limiting factor in regard to tire width: you don't want to go too narrow relative to the rim's width. Here's a link to a table hosted off of Sheldon Brown's website that provides a pretty good set of guidelines for matching tire and interior rim widths: http://sheldonbrown.com/tire-sizing.html#width

Assuming you have a rim that's narrow enough, just about any 26" wheeled tandem can run tires that are as narrow as the ones used on 700c wheeled tandems for teams of similar weights, e.g., 25mm is quite commonly found on go-fast tandems with teams who weigh well into the mid 350lbs range. So, width alone is not as important as using the correct rim/tire combination, sufficient air pressure, and a tire that's durable enough for the road conditions you expect to encounter.